Method for decreasing aging of paper with sulfites and/or bisulfites and product

ABSTRACT

A dried paper web is impregnated with a soluble sulfite and/or bisulfite salt, such as sodium sulfite or bisulfite which salt may be incorporated in an aqueous starch, or starch clay, surface-sizing composition to decrease the aging of the paper.

0 Muted Sta s tet Inventor Richard A. Silberrnan Ridgefield, Conn.

Appl. No. 665,955

Filed Sept. 7, 1967 Patented Nov. 9, 1971 Annlgncc (Mingle-PacificCorporation Portland, Oreg.

METHOD 11 0111 DECREASING AGHNG OF PAPER WHTIHI SUlLlFllTES AND/0R BISUILIFHTES AND PRODUCT 3 Claims, N0 Drawings Casey, Pulp and Paper, 2nd edition, Vol. 11, p.947.

Paper Coating Additives, Tappi Monograph, Series No. 25, 1963, Library ofCongress Catalog Card Nov 62-22434, pages 81- 82.

Primary Examiner-S. Leon Bashore Assistant Examiner-Richard H, Anderson Attorney-Birch, Swindler, McKie & Beckett ABSTRACT: A dried paper web is impregnated with a soluble sulfite and/or bisulfite salt, such as sodium sulfite or bisulfite which salt may be incorporated in an aqueous starch, or starch clay, surface-sizing composition to decrease the aging of the paper.

METHOD FOR DECREASTNG AGllNG OF PAPER WTTIH SlUlLlFllTlES AND/R IBIISULFHTES AND PRODUCT This invention relates to a method for maintaining the brightness of paper and to a surface sizing composition for paper which tends to prevent deterioration and discoloration of paper.

Paper tends to deteriorate by any of a number of means to provide a product of darkened color and/or reduced brightness. Many impurities or chemical additives used in .paper manufacture tend to darken over a period of time probably due to oxidation efiects involving the paper and the chemical additives. Common metallic ion impurities such as iron and magnesium when oxidized, form colors which reduce the brightness and change the color of paper.

These problems are particularly aggravated in papers manufactured from paper pulps containing large amounts of lignin. Such pulps are commonly referred to as mechanical (groundwood), chemi-mechanical and semichemical to distinguish them from the purely chemical pulps. Lignin deteriorates rapidly with natural aging. Heat and ultraviolet light tend to accelerate the natural aging process still further. The problems associated with oxidized metallic ion impurities is also aggravated in high-lignin papers since, in addition to the colored oxidation products, the metal impurities also react with lignin to produce colored complexes.

It is also characteristic of papers containing large amounts of unbleached sulfite or chemi-mechanical pulp prepared by the sulfite-type of cooking process to age more rapidly (with corresponding increased rate of deterioration) than paper manufactured from pure bleached chemical pulp. It is believed that this results from the oxidation of lignosulfonic acid to a form exhibiting a reddish color, to the formation of ferric iron complexes with lignosulfonic acid to produce a pink or red shade material and/or the presence of stilbene-type color compounds which, on oxidation, form a pink coloring material.

ln addition to the foregoing special situations, essentially all papers have a tendency to darken and become embrittled and weak when exposed to high temperatures such as those commonly encountered in duplication machines and preburners on printing presses and in paper driers encountered in the printing processes following the presses. In essence, this type of deterioration is an accelerated aging which can be traced directly to the exposure at elevated temperatures. Similarly, aging of any paper can be accelerated by exposure to sunlight as well as to any conditions normally associated with an increase in oxidation rate.

It has now been found that impregnation of the surface of dry paper, i.e., paper from the dry end of the paper making process containing up to about percent water, with a watersoluble sulfite or bisulfrte, or a mixture of these, will greatly assist in maintaining paper brightness, and greatly retard the aging of the paper from any or all of the sources discussed above.

A convenient method for impregnating the surface of the paper with the sulfite and/or bisulfite is to incorporate the water-soluble salt in the surface-sizing compositions used to treat dry-end paper.

As is well known in the art, it is common to saturate papers with starch or clay-starch solutions or like aqueous colloidal and/or pigmented solutions used for whitening and surface sizing paper. These solutions (including suspensions) can be advantageously employed to aid in the introduction into the surface of the paper and the holding therein of the sulfite and/or bisulfite salts. The appropriate soluble sulfites and soluble bisulfites may be added to the coating composition during its preparation and thus be cooked with the coating or can be added to the coating after it is prepared. The sulfites and bisulfltes can be added to the coating either as dry material or in the form of suitable aqueous solution. The term soluble sulfites and bisulfites is intended to refer to the sodium, potassium and ammonium salts.

The soluble sulfite or soluble bisulfite is applied to the paper at a dosage of from about 5 to lbs. per ton of paper. If significantly less than this amount is employed, the reduced aging characteristics are not sufficiently obtained. On the other hand, if the amount of the mixture exceeds about 20 lbs. per ton of paper, only slight gains are observed.

The present invention does not impose a limit on the opera ble pH range of the treating solution. Any pH which is con sistent with the other characteristics of the paper being treated, including ultimate intended use of the paper, can be employed for the practice of this invention. With the above proviso, a pH in the range of about 5 to 8, and preferably about 6 to 7 has been found quite suitable.

Fresh pulp cooking liquor of the sodium, potassium and/or ammonium sulfite/bisulfite type is suitable for use as the source of sulfite or bisulfite. Typical cooking liquors of this kind may have a pH of about 5.9 to 6 and from about 0.5 to 18 percent sulfite/bisulfite (expressed as S0 The relative amount of sulfite and bisulfite may range from predominantly sulfite to predominantly bisulfite. The cooking liquor referred to in subsequent examples was prepared by dissolving about a half-pound each of sodium sulfite and sodium bisulfrte per gallon of solution.

The soluble sulfite and/or bisulfite may be incorporated in an aqueous starch, or starch-clay, surface-sizing composition. Starch solutions may be prepared by slurrying starch in water and cooking the slurry at, for example, 190 F. for about 20 minutes. These starch solutions contain about 3 to 15 percent by weight of dissolved solids. The viscosity of the solution can be varied depending on the manner in which it is to be applied. Commonly, a size press is employed, but these techniques are well known in the art. The clay-starch or pigment-starch solutions are similarly prepared. The clay or pigment may, however, be added to the starch solution at any stage of preparation, i.e., before, during or after cooking. Typical clays used in the process include the kaolin clays. ln general, the surface sizing solution is applied in an amount of about 160 gallons per ton of paper treated. This is usually increased by the amount of sulfite solution added to the usual sizing solution.

The following tests and/or methods were employed for the evaluation of properties and characteristics of treated papers:

Brightness Determined according; to TAPPl testing standard T 453 ts-63 Bursting Strength Determined according to TAPPI test ing standard (Mullen) T 403 m-53 Tearing Resistance Determined according to TAPPI testing standard (Tear) T 414 ts-65 Brightness reversion Determined as the difference between the initial brightness of a test specimen and the faded brightness, i.e., the brightness after subjecting the specimen to a treatment which tends to fade the specimen.

Paper S0 content Determined by extracting 5 grams of sample with 150 ml. of distilled water, measuring pH, acidifying with sulfuric acid, addition of starch-potassium iodide indicator, and titrating with 000125 N K10 to the first lasting color change. The number of ccs of KIO solution required is an indication of the amount of SO, in the sample.

The effect of aging on the several characteristics of the paper can be indicated by accelerated aging of the paper with comparison of the properties before and after aging. In the case of brightness, these effects are referred to as fading and/or brightness reversion. Oven. or heat aging is effected by the general accelerated method described in TAPPl tests T 453 ts63 and RC-280, modified to employ an oven temperature maintained in the range 215 to 220 F. with the specimens held in the oven for 6 hours. Light aging is effected by exposing the test specimens to a bank of six G.E. sun lamps for 6 hours while maintaining the ambient temperature below F.

Gas aging is effected by hanging the test specimens in a stream of cooled combustion gases from a bunsen burner operating wide open. The gases are cooled to a temperature starch solution was applied to the paper in a size press and below 100 F. brightness measurements made before and after aging treatin the examples which follow, paper weight refers to the ment. in the first series, the starch solution contained no addiweight ofa ream made up of 24 inch X36 inch sheets, unless lives. In the second and third series, the starch solution conotherwise indicated. All of the tests and aging were conducted tained sodium sulfite-bisulfite and was prepared by adding 1 l as described above unl s O h r indicatedgallons of cooking liquor, as described previously, to 160 gallons of regular starch solution. The final solids content of the EXAMPLES 1 13 treating solution was 7.6 percent. The brightness data, etc. ob-

Stareh solution was applied to white bl paper f 37 tained for each series were averaged. Each average represents pound weight and to white form paper of 46 pound weight. five brightness tests per sheet for 36 evaluations, or a total of The papers were d up f mixtures f h p h i l I80 brightness readings. The averaged data are set forth in bleached kraft and bleached sulfite pulp. Part of the paper was table also treated with sodium sulfite-bisulfite (fresh cooking TABLE III liquor) in accordance with the practices of the present invenl5 I ss 1055 p r Orlglnal after aging bycharacteristics tion. After treatment, some specimens were sub ected to acsuifiwbisulfite bright, -.v celerated aging which, in these examples, was conducted for mated Gas Light Heat PM so? 24 hours at 130 C. A comparison of the physical properties of h I l the paper before and after aging, with sulfite-bisulfite treat- 33:3 gjg :28 2;; hi, ment and without, is provided in table I. 70.0 1.0 3.0 1.75 0.3 100 TABLE I EXAMPLE 24 Paper Sulfite- I Weight, bisulfite Origmal ve -age al O -aged Following the foregoing procedures, 37 pound offset printlbs. treated tear tear mullen mullen mg paper containing fluorescent dye and other brightners was 23 g -g ig-g -treated in the manner described above. The sulfite-bisulfite 40 22 22: treatment of the present invention provides improved 40 as 21.5 10.3 42 38 2L9 18.9 brightness, significantly improved brightness reversion gg-gg 233g 5%.}; characteristics and significantly reduced the loss in physical 292 2 properties of the paper on aging.

8 0-36 29. 2 13.9 54-55 54-54 28.1 25.0 EXAMPLES 25-27 56-58 50-56 29. 5 27. 1 56-56 52-54 26. 0 23.2 In an attempt to evaluate the amount of sulfite-bisulfite ad- 50-54 50452 5 7 dition required to obtain the advantages of the present inven- EXAMPLES 14 20 tion, a starch solution was prepared which contained five gallons of cooking liquor per 160 gallons of starch solution rather Followmg the foregomg Procedures the effect of Soluble than the 11 gallons used in previous examples. The paper sulfite-bisulfite treatment on brightness was determined durtreated was 4 pound white f paper and the tests were ing Continuous Processing of white form P P After Continu' conducted in the same manner described previously with the ous operation was established with a standard starch surface- 4c er te ted with just the starch solution and with the starch sizing Solution, fresh Sodi m sulfitei lfite C king liqu r sulfite-bisulfite solution. The results of these tests are set forth was mixed with the starch solution in a proportion of 1 1 gal; in table IV,

TABLE IV Starch Brightness loss characteristics after aging by Original Aged Mullen Tear Sulfite bisulfite Original additives pH S02 brightness Gas Light Heat pH S0 pH S02 Porosity Original Aged Original Aged No 6.5 0.2 76.2 2.5 5.0 8.4 5.7 6.0 5.8 2.5 21 23.0 28.3 52-58 -54 Yes 6. 1 79. 6 76. 3 2.0 4. 1 6. 3 6. 2 66. 6 6. 4 37. 4 30 24. 3 26. 0 48-62 46-52 Yes 0 1 79.6 74.6 2.0 4.8 6 0 6.2 51.8 6.1 28.0 30 29.0 30.5 56-58 56-60 lens of cooking liquor per 160 gallons of starch solution. This EXAMPLES 28-38 corresponded to a sulfite-bisulfite addition to the paper in the range of about 11 to 12 pounds per ton. The brightness and brightness reversion on accelerated aging were determined before and during sulfite-bisulfite treatment. The amount of sulfite-bisulfite picked up by the paper and contained in the starch was monitored regularly and reported as ccs of 0.00125 N K10 under the heading S0 The results ofthese 0 tests are provided in table ll.

Three surface-sizing solutions were prepared. The first consisted ofa 7.6 percent starch solution. The second consisted of the same starch solution but contained, in addition, sufficient clay (kaolin) to provide We pounds of clay per 37 pound ream of 24 inch X36 inch paper. The third solution was the same as the second solution but contained, in addition, 10 gallons of sodium sulfite-bisulfite cooking liquor for each ton of paper to be treated (Le, 10 gallons of liquor per 160 gallons of starch TABLE M solution). 37 pound offset printing paper was treated in a sizstamh characteristics L055 in brightness ing press with the three sizing solutions. The results are set Original after aging by Paper Percent biightforth in table V. For the heat aging in the tests the oven was {ill solids S0; ness Light Gas Hear pH SO; maintained at 5 p 74. 8 4. 9 1. 3 3. 0 5. 7 8. 2 6.0 8.2 0.2 74.9 4.0 1.3 3.0 5.7 7.4 EXAMPLES 39-44 75.2 5.1 1 5 3.0 5.8 7. 5 U 2-3 5.2 2-2 fag-3 Paper prepared from lOO percent full bleached chemical 1 3 2 151 g 4:1 1: 5 2: 6: 5 157: 9 pulp (bleached kraft and bleached sulfite), which had already 155-4 5 5 150-0 been surface sized with starch, was divided into two batches.

* The firstwas treated solely with water. The second was treated EXAMPLES 2143 with an aqueous solution of sodium bisulfite. Brightness tests Following the foregoing procedures, three series of tests were then conducted on the specimens before and after aging were conducted on 28 pound white form paper. A regular and the change in brightness determined. In a significant TABLE V Brightness loss i I after aging by- Original Heat aged Original Heat aged r gma Coating brightness Gas Light Heat pH S02 pH S02 Mullen Tear Mullen Tear Starch 73.5 4.8 4.0 4.7 6.2 460 6.3 160 20.5 28-36 22.4 :22-28 Stareh-clay- 74.2 4.1 4.0 4.8 20.3 32-44 22.9 26-36 Do... 73.3 3.9 3.0 4.5 21.7 28-3 22.6 :26-32 Do 73.1 4.3 3.6 3.7 21.5 26-36 19. 3 26-32 Do... 72. 5 4. 4 4. 1 4. 6 22. 5 32-36 22. 7 '2632 Do. 72.2 4.0 3.5 4.5 20.7 30-36 20.2 26-30 Starch-elay-suliit 73.3 3.3 2.9 4.0 20.5 28-34 21.2 22-26 D 73.2 3.5 3.3 3.8 22.5 24-34 22.0 26-28 Do. 73.2 3.1 2.8 3.7 20.5 32-34 21.0 22-30 Do. 73.4 2.4 2.8 3.6 22.0 28-32 23.2 28-30 [)0 73.4 2.4 2.0 4.5 21.5 32-34 21.0 24-30 number of cases the brightness actually increased on aging. 72.3 2.0 2.6 1.1 '|h.d d .hl v I t 74.0 2.1 3.3 4.. e .1 .1 are summarize m a e wit a p us sign in ma 7&3 Z5 33 0 11' br' 73.7 2.7 3.5 3.0 mg n mere iscin ightness and .1 negative sign indicating 3 undo".-

H 73" 35 3,5 3.5 hfltlhllwhe Stareh-eiay-bisulfite... 75.0 3.1 2. 7 3.0 i ..do 74.5 1. 0 3.0 4.1 .ltL Stareli-bisuilito 77.7 2.3 3.1 3.7 3 28.. .Stutci...... 76.5 1.2 2.!) 7.4 LXAMPLES 45 50 23. ..(lo 77.0 1.3 3.3 0.5 221. (lo 78.5 3.1 4.2 6.4 lOllOWlIlg the foregoing procedure, tests were conducted 2s Starch-clay.-. 76.6 2.2 3.8 3.8 23-. Starch 75.1 2.3 3.1 6.5 with 27 pound tablet paper. The starch solution employed starclbbisulmuwu 76.1 2.0 M 10 contained 7.2 percent solids. To 160 gallons of the starch solu- 5.1

. g t tron were added 10 gallons of sod1um sulfite-bisulfite cooking 8 bisulm (DH 5 liquor to provide a treating solution containing 3728 p.p.m. of 28 s zrghisulfi e (p11 76. 5 2.1 3. 4 2.8 S0 The results of the test are set forth in table VII.

TABLE VI Brightness change on aging by Light Gas Heat Water Bisulfite Water Bisuifite Water Bisulfite Original brightness treated treated treated treated treated treated TABLE VII Thus, the process of the present invention involves the sur- B i ht 103s :face treatment of paper at a stage subsequent to the pulp de- Origmal j i i i 23: riginal 40 watering and web formation stages; and subsequent to the ini- Sullite-blsullitu brightness Light Gas Heat pH so, a pressing and partial drying of the paper. it may. ho e ML h 72 U 4 7 1.8 4.6 5- 9 328 precede final drying, e.g., drying to final water content and/or 73.0 3.2 1.4 3.8 6.4 4,440 calendering, and may be conducted in connection with, or gig i'g 2'5 g-g @228 subsequent to surface sizing (e.g., tub sizing as distinguished 72.6 4. 0 2.1 4.8 5.8 '820 from engine sizing). The product is a paper having a surface 72.3 4.7 1.3 4.5 5.6 548 EXAMPLES 51-68 Following the foregoing procedures, mill average control tests were conducted during the production of 46 pound white paper produced from 30 percent bleached kraft and 70 percent chemi-mechanical bisulfite pulp, and of 28 pound form paper produced from 45 percent bleached kraft and 55 percent chemi-mechanical bisulfite pulp. in the tests, the starch was normalized at 8 percent solids. When clay was included, it was at a concentration of about a fifth ofa pound per gallon of size press solution, such that it was applied in an amount of about 50 pounds per ton of paper. The sulfite/bisulfite liquid was incorporated in an amount of about a tenth ofa pound (as sodium sulfite) per gallon of size press solution or approximately 1 1 pounds per ton of paper treated. The results of the tests are set forth in table Vlli.

impregnated with one or more of the specified sulfites and bisulfites and will be characterized by significantly retarded aging.

2. The process of claim 1 wherein the sulfite salt is a sodium salt.

3. As an article of manufacture, a dry paper web having a surface impregnated with starch and a sulfite salt selected from the group consisting of sodium. potassium, and ammonium sulfites and bisulfites, and mixtures thereof. in an amount of from about 5 to about 20 pounds of suliite per ton ofpaper. 

2. The process of claim 1 wherein the sulfite salt is a sodium salt.
 3. As an article of manufacture, a dry paper web having a surface impregnated with starch and a sulfite salt selected from the group consisting of sodium, potassium, and ammonium sulfites and bisulfites, and mixtures thereof, in an amount of from about 5 to about 20 pounds of sulfite per ton of paper. 